The present invention relates to a drive circuit for a piezoelectric pump used in a cooling device for cooling a heat-generating body of an electronic component, and to a cooling system that uses this drive circuit.
Processors for high-speed processing are used in, for example, notebook computers, and cooling devices are required for reducing the rise in temperature caused by the heat generated by this equipment. One such cooling method of this type in the prior art is the water-cooled method that employs a piezoelectric pump as described in JP-A-2001-355574.
When using a piezoelectric pump, an alternating current at a voltage of approximately 100V is required as the drive voltage to produce a large displacement when deforming the piezoelectric material that is used in the piezoelectric pump. In addition, the drive frequency is a low frequency of several tens to several hundred Hz due to the response of valves that operate in accordance with the displacement of the piezoelectric pump.
The power supply in electronic equipment that uses a cooling device that employs a piezoelectric pump has a low voltage such as +5V, and an inverter circuit is therefore necessary as the piezoelectric element drive circuit of the piezoelectric pump to produce power having low-frequency at a high voltage from the low-voltage power supply.
Normally, a low-frequency transformer is used as the above-described inverter circuit, but the use of a low-frequency transformer increases the size of the device and necessitates large packaging space and thus raises problems when used in mobile equipment that requires a compact and thin form. As a solution to this problem, JP-A-2002-339872 discloses a drive method for driving a piezoelectric pump. FIG. 1 is a schematic block diagram showing the configuration of the circuit provided in JP-A-2002-339872, FIG. 2 is a block diagram showing the configuration of this circuit in greater detail, and FIG. 3 is a waveform chart of each part showing the operation of this circuit.
Explanation next regards the circuit that is proposed in JP-A-2002-339872 with reference to FIG. 1. In FIG. 1, oscillator 111 generates a first clock signal and a second clock signal having prescribed frequencies. The first clock signal is a signal of a frequency that matches the drive frequency of piezoelectric element 101 that drives the piezoelectric pump, and the second clock signal is a carrier signal of higher frequency than the first clock signal that is set by taking into consideration the amplification efficiency of amplifier 102. In this circuit, the frequency of the first clock signal is set to 50 Hz, and the frequency of the second clock signal is set to 14 kHz.
Modulator 112 uses the first clock signal that matches the drive signal of piezoelectric element 101 to perform AM modulation of the carrier wave that is constituted by the second clock signal to produce a modulated wave signal. This modulated wave signal is applied as input to amplifier 102 to realize signal amplification. The modulated wave signal following amplification is applied as input to demodulator 113 whereby a modulated signal (the pump drive signal) of the same frequency as the first-clock signal that has been amplified is extracted and applied to the electrodes of piezoelectric element 101.
According to the above-described configuration, amplifier 102 amplifies the second clock signal, which is of a higher frequency than the first clock signal. In other words, the second clock signal is amplified and a modulation process implemented to produce a desired drive signal without directly amplifying the first clock signal, and because the low-frequency signal is not directly amplified, the problems of increased complexity, size, and cost of amplifier 102 can be avoided. In particular, JP-A-339872 shows that a more compact and lighter component can be realized by using a high-frequency transformer as an amplification circuit.
Explanation next regards the details of the operation of this device with reference to FIGS. 2 and 3.
In FIG. 2, 114 is a frequency divider, 115 is a NOT circuit, 116 is the first AND circuit, 117 is the second AND circuit, 102a is the first amplifier, 102b is the second amplifier, 113a is the first demodulator, and 113b is the second demodulator.
Oscillator 111 generates a second clock signal of 14 kHz. This second clock signal is branched and applied as input to each of frequency divider 114, first AND circuit 116, and second AND circuit 117. The signal that is applied as input to frequency divider 114 is frequency-divided to the drive frequency of piezoelectric element 101 to produce a first clock signal of 55 Hz. This first clock signal is branched, one portion being directly applied as input to first AND circuit 116 and the other portion being applied as input to second AND circuit 117 by way of NOT circuit 115. These AND circuits 116 and 117 implement AM modulation. The two modulated wave signals are respectively applied as input to first amplifier 102a and second amplifier 102b to undergo amplification and then drive piezoelectric element 101 by way of first demodulator 113 and second demodulator 113b, respectively.
FIG. 3 shows the signal waveforms at points A, B, C, and D in FIG. 2 and the signal waveform that is applied to the piezoelectric element. The signal at point A is the second clock signal, which is the signal generated at oscillator 111 as shown in FIG. 3(A), and is the carrier wave in the modulation process. The signal at point B is the first clock signal obtained by the process of frequency-dividing the second clock signal and is a signal of the same frequency as the pump drive frequency. This signal is the drive signal that precedes modulation in the modulation process. In addition, a signal of the opposite phase of the second clock signal is applied as input to second AND circuit 117. The signal at point C is the modulated wave obtained by modulating the first clock signal with the second clock signal as the carrier wave as shown in FIG. 3(C). The signal at point D is the modulated wave obtained by modulating a signal of opposite phase of the first clock signal with the second clock signal as the carrier wave, as shown in FIG. 3(D).
The differential of the signal at point E, which is a signal obtained by demodulating the modulated wave by first demodulator 113a, and the signal at point F, which is a signal obtained by demodulating the modulated wave by second demodulator 113b, is applied as input to piezoelectric element 101, whereby piezoelectric element 101 is driven.
The first problem to be solved by the present invention is the generation of large vibrational noise from the piezoelectric pump and the consequent inability for application to devices that are used in environments in which quiet operation is desired. Although the drive waveform of a piezoelectric pump is a frequency that is lower than audible frequencies, this vibrational noise occurs both because, the drive waveform includes a harmonic frequency component within the audible frequency band, and because the drive waveform is not a sine wave. Undesired noise is produced by the vibration of the harmonic component.
The second problem is the inability to start the circulation of liquid due to inadequate pressure from the piezoelectric pump at the time of activating the power supply. This problem occurs because temperature changes in the electronic equipment or pressure changes in the liquid passages cause cohesion of, for example, oxygen that is contained in the liquid and the consequent occurrence of bubbles in the liquid passages of the coolant. These bubbles remain in the pump chamber of the piezoelectric pump and thus absorb pressure and reduce pressure that is transmitted from the pump to the liquid.
The third problem is the pointless consumption of power that results from the operation of the piezoelectric pump and the drive circuit even when heat is not being generated by the heat-generating body, i.e., the electronic component that is to be cooled.